1. Field of the Invention
The present invention relates to a probe for sensing temperature and/or pressure which is particularly useful in an apparatus for and the process therefor. More particularly, the present invention relates to such a probe for sensing pressure which is useful in an apparatus for and process of pasteurizing or sterilizing and subsequent cooling of foodstuffs within sealed thin-walled containers, wherein the pressure within the apparatus is controlled and maintained so as to be substantially equal to or substantially at a predetermined differential from the pressure within the thin-walled containers during pasteurization or sterilization and the subsequent cooling thereof.
2. Prior Art
There have been numerous devices and processes proposed in the prior art which have attempted to preserve material which has been packed in hermetically sealed containers. One of the most perplexing problems faced in developing such devices and processes has been maintenance of the pressure inside the pasteurizer or sterilizer so as to counteract pressure developed inside of the containers during the pasteurizing or sterilizing process itself without the influence of changing the thermal conditions thereof.
A method for cooking or preserving material packed in hermetically sealed containers in which the material is subjected to sterilizing temperatures is known from U.S. Pat. No. 1,881,855 to George W. Mullen entitled "Method for Preserving" and granted Oct. 11, 1932. It was recognized that internal pressure within the containers in excess of external steam pressure, if not counterbalanced, may distort metallic containers or burst glass containers. In order to overcome the difficulty, it was proposed to entrap a quantity of air, preferably at atmospheric pressure, in the retort within which the containers were to be placed, and to subject the air to the same heating medium as the containers. No provisions were made to add air under pressure to the entrapped air or to vent air or steam from the retort to provide real time control of pressure differential.
A process of heating and sterilizing food products packaged in hermetically sealed containers is known from U.S. Pat. No. 3,215,538 to Pietro Sader et al. entitled "Process for Heating and Sterilizing Food Products Packaged in Hermetically Sealed Thin Walled Containers" and granted Nov. 2, 1965 similar to the method disclosed in the patent to Mullen, supra. Rather than simply entrapping air, it was proposed to increase the pressure within the autoclave (retort) by a pressurized fluid to balance increased pressure within the containers as the containers are heated and, as the containers are cooled, discharging the pressurized fluid t balance the decrease in pressure within the containers. No provisions were made to measure the actual pressure within the thin-walled containers nor to use such a measurement, in conjunction with a measurement of pressure within the autoclave (retort),.to provide real time control of pressure differential.
A process for cooking, baking or sterilizing foodstuff within a retort and contained in sealed, imperforate, flexible packages formed of film or sheet material is known from U.S. Pat. No. 3,531,300 to Greenberg et al. entitled "Process for Heat Treating Food Sealed Within Flexible Containers" and granted Nov. 17, 1964 in which an effort is made to counteract expansion of the packages beyond a volume limit by providing sufficient external pressure on the packages. No provision is made to determine the actual pressure within the packages and the actual pressure within the retort, nor to control the pressure differential in real time.
Of interest as general background prior art are a number of U.S. Letters Patents identified as follows:
______________________________________ U.S. Pat. No. Inventor(s) Granting Date ______________________________________ 3,816,633 Schack June 11, 1974 3,986,832 Smorenburg October 19, 1976 4,088,444 Byrne May 9, 1978 4,164,590 Mencacci August 14, 1979 4,340,610 Nioras July 20, 1982 4,346,650 Zaitsu August 31, 1982 4,468,135 McCain et al. August 28, 1984 4,468,968 Kee September 4, 1984. ______________________________________
Processes and apparatuses have been proposed to avoid the problem of distorting containers during pasteurizing or sterilization of the contents by resorting to heavy thick-walled containers fabricated from material, such as metal, which is costly and not conducive to marketing. Additionally, use of such containers requires special equipment for punching holes therethrough so that the appropriate sensing equipment may be inserted thereinto. Such equipment is costly, requires additional floor space and labor and slows production.
In recent years, however, there have been great developments in the field of plastics. Despite these recent developments, no one has yet devised an apparatus or process for pasteurizing or sterilizing foodstuffs within thin-walled containers in a chamber (retort) and subsequent cooling thereof which utilizes techniques for controlling the pressure within the chamber on the basis of real time actual pressure measurements of pressure within a container or containers and the chamber.